Process for forming visible images by electron beam recording



United States Patent 3,495,266 PROCESS FOR FORMING VISIBLE IMAGES BY ELECTRON BEAM RECORDING Horatio Serafino Stillo, White Bear Lake, Minn., assignor to Minnesota Mining and Manufacturing Company, St. Paul, Minn.. a corporation of Delaware No Drawing. Original application Sept. 23, 1963, Ser. No. 310,922, now Patent No. 3,425,867, dated Feb. 4, 1969. Divided and this application May 9, 1967, Ser. No.

Int. Cl. G01d 9/02 US. Cl. 346-1 4 Claims ABSTRACT OF THE DISCLOSURE This is a division of application Ser. No. 310,922, filed Sept. 23, 1963, now Patent No. 3,425,867.

The present invention relates to media for recording intelligence, particularly for the recording of intelligence by means of an electron beam, and to the process in which such media are used.

Cathode ray tubes with phosphor screens having a visible response essentially instantaneous with electron beam impingement have been commonly used for transducing transmitted intelligence in electrical form into a visual form. However, the image on the phosphor screens of such tubes is transitory and cannot be stored for any length of time without auxiliary means for providing a permanent record. Numerous efforts have been directed to recording permanently the transitory images on cathode ray tubes by various electronic and optical photographic methods. Special tube faces have been suggested to permit the use of electrostatic development of special papers passed continuously over the tube face, but these tube constructions frequently have produced records with poor resolution due to various forms of distortion associated with the system. The recording of a light image on a medium containing an acid-base type indicator dye and a halogenated compound which produces hydrochloric acid under the influence of light, has been described in the literature. However, applicant has found that such media, when placed under vacuum conditions for direct electron beam impingement, tend to lose volatile reactants, e.g. the relatively low molecular weight halogenated compounds.

It is an object of this invention to provide an electron beam recording medium which can be used under vacuum conditions and which can be activated with direct elecice A further object of this invention is to provide means for recording facsimile or electrical intelligence.

Other objects and advantages will be apparent from the following description.

In accordance with this invention the above and other objects are realized by providing a recording media which comprises an electrically conductive substrate on which is contiguously superimposed a layer containing an acid sensitive indicator in basic form and a highly halogenated polymeric binder. Although the indicator is generally homogeneously disposed throughout the halogenated polymeric binder, and is preferably dissolved therein, it .may also be provided as a localized coating on or in the top surface of the polymeric binder.

Any acid sensitive indicator or pH sensitive system capable of effecting a color change at a pH of from about 1.5 to about 7 may be employed in its basic or unreacted form, the more preferable materials being those oil soluble materials which alter their color value in the pH range of 2 to 4 and have relatively low volatility, e.g. a boiling point above about C. at atmospheric pressure. One preferred class of useful indicators includes those dyes which become quinoid in the presence of acid, such as the amino azo dyes, e.g. dimethyl aminoazobenzene (Methyl Yellow, Cl. 19), characterized by their good sensitivity and, in the case of Congo Red A, ability to form very stable images. Color bases of diaryl and triaryl methane dyes (e.g. Auramine) can also be used. Other illustrative dyes include the azines (e.g. Natural Red), phthaleins (e.g. Thymol Blue), hydroxyazo dyes (eg Salochrome Yellow 3G), etc. The oil soluble dyes are generally preferred because the presence of water solubilizing groups (e.g. acyl groups and their salts) tends to reduce the electron beam sensitivity of the film. Solubility of the indicator in the highly halogenated polymeric binder normally permits optimum results with a minimum concentration of indicator. Various other systems, including those in which two or more materials react, e.g. by coupling, in an acid environment to form a material of another color, with or without oxidation, heat, etc. are within the scope of this invention. Where oxidation assists the reaction, exposure to the atmosphere frequently will accelerate the reaction, and in some instances the inclusion of an oxidizing agent in the electron beam sensitive layer can be of value for this purpose.

The highly halogenated polymers which liberate hydrogen halide upon electron beam bombardment should be normally solid and of sulficiently high molecular weight to prevent their volatilization (i.e. above 1000 preferably above 10,000 number average molecular weight), be film forming, and contain, in addition to hydrogen, from about 25 to about 73 weight percent of labile chlorine or bromine. For ease of coating the electrically conductive substrate the polymers desirably are soluble in conventional organic solvents such as tetrahydrofuran, acetone, 2-butanone, methyl ethyl ketone, etc., although other solvent systems can be used for the more diflicultly soluble polymers such as polyvinyl chloride and polyvinylidene chlorine. Solubility can be adjusted to some extent by employing copolymers, a balance being achieved between halogen content and copolymer solubility. Vinylidene chloride copolymers with such monomers as the aliphatic acrylates (e.g. n-butyl acrylate, methyl acrylate, ethyl acrylate, hexyl acrylate, methyl methacrylate, beta-chloroethyl acrylate, etc.), acrylonitrile, vinyl chloride, vinyl acetate, vinyl butyrate, etc. are preferred highly halogenated polymer systems. Ethylenically unsaturated monomers with a high halogen content, such as l,1,3,3,3-pentachloropropene 1, fluorotrichloroethylene, 1,1 difluoro 2,2 dichloroethylene, trichloroethylene, etc., copolymerized with vinyl or vinylidene chloride or bromide or with the aliphatic acrylates can also be employed. Halogenated aromatic polymers are considerably less effective than the halogenated aliphatic polymers, although the copolymerization of a suitable halogenated aliphatic monomer with an aromatic monomer (e.g. styrene, vinyl toluene, vinyl carbazole, etc.) selected for its solubility characteristics is suitable. With the preferred vinylidene chloride polymers the chlorine concentration ranges from about 25 to about 73 percent, preferably from about 40 to about 70 percent by weight. With the vinyl chloride polymers the chlorine concentration ranges from about 35 to about 55 percent, preferably from about 20 to about 55 percent by weight of the polymer. Although the halogenated polymers are desirably deposited from solution as a film on the electrically conductive substrate, they may also be deposited from a latex or intimate dispersion. With those polymers which tend to decompose slowly in the presence of ordinary light and atmospheric oxygen, anti-oxidants and other stabilizers may be added to improve good storage life.

Since the highly halogenated polymer serves as a relatively non-volatile source of hydrohalic acid, no other brominated or chlorinated compounds which liberate acid under electron beam exposure are required in the film. In fact, the monomeric or non-polymeric halogenated compounds are undesirable not only because of their higher volatility in vacuum but also because of their frequency encountered sensitivity to moisture, air oxidation and photodecomposition under visible light and their adverse effect on the physical properties of the film, such as by dilution of the polymeric vehicle. Materials which are highly sensitive, such as silver halide, zinc oxide, etc., are also undesirable, since the media construction should be essentially stable to visible light. The slight fading of certain indicators upon extended light exposure, is however, unobjectionable. These problems are eliminated and the reliability of the recording sheets is improved by excludin non-polymeric sources of hydrogen halide and visible light sensitive materials.

The media of this invention may be prepared by mixing a minor amount of the acid sensitive indicator system with a solution of the highly halogenated polymer and coating the resulting admixture as a thin film, i.e. usually from 1 micron to several mils, preferably from about 2 to about microns, onto the electrically conductive substrate. Aluminum foil, metal coated plastic, conductive papers, etc. can be used for the electrically conductive substrate. If a transparency is desired or if it is desired to view the image through the backing, a thin, light transmissive aluminum vapor coated plastic (e.g. polyethylene terephthalate) or a conductive glass (e.g. Nesa glass) substrate may be used in conjunction with a light transmissive, electron beam sensitive coating. Many of the highly halogenated polymers may be made even more relatively light transmissive in the form of a thin film. For each equivalent weight of acid sensitive indicator from about 1 to about 1000 acid equivalents of the halogenated polymer are employed, although the ratio of these ingredients varies with the particular indicator system, and its acid sensitivity, which is employed. Other additives, e.g. plasticizers, oxidizing agents, etc. may be incorporated into the electron beam sensitive coating, provided they do not serve to liberate acid under the electron beam. Additional films or coatings may be provided on the electron beam sensitive layer to protect it from abrasion, etc., provided they are relatively transmissive to the electron beam.

With the media described above a color change is generally observed in the sensitive coating immediately upon electron beam impingement or shortly thereafter upon exposure to air, thereby providing a visible record. In some instance, when the acid sensitive indicator is reversible, as with the acid-base indicator dyes, the image can be erased by heatin the media to about 100 C. to 150 C. for approximately 30 seconds, the color change being probably due to the volatilization of the acid and an increase in effective pH of the media. Erased media of this type can be reused for electron beam recording, although subsequent depletion of the polymeric acid source eventually reduces the efficiency of the recording.

It is also within the scope of this invention to impinge the electron beam on a media comprising the highly halogenated polymer film on an electrically conductive substrate, leaving the indicator out of the polymer film. After the exposure to the electron beam, the liberated acid in the imaged areas can be subsequently developed by contacting the exposed surface with the acid sensitive indicator system. A separate development roller or bath may be used for this post development step, or a second film construction incorporating or carrying the indicator can be brought intocontact with the exposed surface of the media. One advantage of a post development using an indicator-containing film is the ability to prepare multiple copies.

If the electron sensitive layer can be supported on and contiguous with an electrically conductive roll or plate during beam exposure, the electrically conductive substrate can be eliminated as an integral part of the recording media construction. However, good electrical contact between the electron sensitive layer and a conductive backing, normally maintained at ground potential, is best achieved with an integral or unitary media construction including an electrically conductive layer contiguous with the electron beam sensitive layer.

Although foregoing description has been directed to electron beam recording, the media described are also sensitive to other forms of relatively high energy irradiation which causes the liberation of hydrohalic acid from the highly halogenated polymer, and alpha, beta and gamma radiation as well as X-rays and ion particles and ultraviolet light below 3,000 angstroms may also be employed at appropriate energy levels.

Since ultraviolet light below 3,000 angstroms is similar to an electron beam in its effect on the recording media of this invention, a simple standard test procedure has been developed to assist in the selection and definition of the preferred highly halogenated polymers and indicator systems for use in the electron beam sensitive layers of the media. The suitability of a film forming halogenated polymer is determined by adding 5 milligrams of Congo Red A to 1.0 milliliter of a 20 weight percent solution of the polymer in a suitable solvent, such as tetrahydrofuran. This solution is then knife coated onto a cellulose acetate, polyethylene terephthalate or glass backing to provide a dry film of 0.1 mil thickness. A sample of this dry film is placed at a suificient distance from an ultraviolet light source to provide about 0.08 watts per square centimeter of radiant energy of 2000 to 3000 angstroms wavelength. The sample is irradiated for a period from 2 to 30 seconds. Generation of a blue color indicates a halogenated polymer containing labile halogen useful in the electron beam recording media of this invention. The same standard test procedure is modified for selection of a suitable acid sensitive indicator by using a 20 weight percent solution of vinylidene chloride-acrylonitrile copolymer 10 mol ratio) and 5 milligrams of the acid sensitive indicator system, a strong color change after the ultraviolet exposure indicating a useful indicator for the electron beam recording media.

The following examples will serve to illustrate the recording media and processes of this invention.

EXAMPLE 1 To 4 ml. of a 20% (by weight) solution of vinylidene chloride/n-butyl acrylate copolymer (90/10 mol ratio, approximately 60% chlorine) in tetrahydrofuran, was added 10 mg. of Congo Red A dye (unsulfonated Congo Red prepared from tetrazotized benzidine and l-naphthyl amine) and 4 drops of cyclohexanone. This solution was used to prepare a film of 0.1 mil dry thickness when it was knife coated onto 3 mil aluminum vapor coated polyethylene terephthalate film.

An electron beam recording made with a scanning rate of 60 fields/sec, kv., 5 microampere beam current, 10 micron spot size and 50 microsecond/ scan line (262.5 lines/field) produced a direct print out blue image on an orange background. The individual scan lines were distinctly visible under a microscope. The change in optical density between background and image was about 0.5 optical density units.

Congo Red A has the following structure:

NH: NH2

EXAMPLE 2 To 4 ml. of a (by weight) tetrahydrofuran solution of a polymer of vinylidene chloride/n-butyl acrylate (92/8 mol ratio, approximately 62% chlorine) was added 20 mg. of Congo Red A dye and 4 drops of cyc ohexanone. This solution was used to prepare a film of 0.1 mil dry thickness by knife coating onto 3 mil aluminum vapor coated polyethylene terephthalate film. When it was recorded as above a similar image was obtained having an optical density difference of 0.6 between background and image.

EXAMPLE 3 Films prepared and recorded as above using 20% tetrahydrofuran solutions of vinyl chloride/vinyl acetate copolymer (43% by weight chlorine); vinylidene chloride/acrylonitrile copolymer (54% chlorine); vinylidene chloride/acrylonitrile copolymer (67% chlorine); tosylated polyvinyl alcohol; chlorinated polypropylene; copolymers of vinylidene chloride with any of methyl acrylate, B-chloroethyl acrylate, vinyl chloride, vinyl acetate or vinyl butyrate, into each of which 1-20 mg. of Congo Red A dye were added (per 200 mg. polymer) yielded direct print out images. The optical density difference between image and background varied depending on the specific polymer and the amount of dye used. However, the films all gave difierences lying in the range of 0.1- 0.3 optical density units.

EXAMPLE 4 To 4 ml. of a 20% (by weight) solution of a polymer of vinylidene chloride/n-butyl acrylate (90:10 mol ratio; approximately 60% chlorine) in tetrahydrofuran was added 20 mg. of 4-phenylazo-diphenyl amine and 4 drops of cyclohexane. This solution was used to prepare a film of 0.1 mil dry thickness when it was knife coated onto 3 mil aluminum vapor coated Mylar film.

An electron beam recording made with a scanning rate of 60 fields/sec, 10 kv., 5 microampere beam current, 10 micron beam spot and S0 microsecond/scan line showed a direct print out, brown image on a pale yellow background. The individual scan lines were distinctly visible under a microscope. The change in optical density between image and background was about 0.5 optical density units.

EXAMPLE 5 Using a vinylidene chloride/n-butyl acrylate copolymer (about 60% chlorine) 'and the same procedure for coat ing and recording as outlined above in Example 1 the following amino azo dyes were tested:

phenyl mercuric acetate.

6 EXAMPLE 6 A recording medium having outstanding characteristics was prepared by adding 0.030 g. of Congo Red A to 3.5 ml. of a solution of 5.00 g. of a copolymer of 87% vinyl chloride and 13% vinyl acetate dissolved and diluted to 100 ml. with methylethyl ketone. To this solution was added 1.5 ml. of methylethyl ketone, 4.0 ml. of n-amyl acetate and 1.0 ml. of 1,l,2,2-tetrachloroethane (these last two solvents are added to decrease the evaporation rate and to obtain unblushed coatings). The resulting clear red solution was then dip coated onto aluminum foil at a rate of 43 ft. per second. The coated foil was pulled vertically into a 1 /2 inch diameter tube where it was allowed to dry. Solution 1.25 to 5.00 g. per 100 ml. solvent gave dry coating thickness of 3 to 5 microns. The dried, coated foil was then evaluated for sensitivity using an electron gun having an unmodulated, non-sweep beam from about 2,000 to 15,000 volts and beam currents up to about 50 microamperes. The spot size was about 0.50 mm. A sensitivity of about 0.1 microamperesecond/mm. was obtained. This corresponds to a recording sensitivity of about 4 megacycles per second.

Media using other concentrations of polymer and Congo Red A were prepared and successfully tested for electron beam sensitivity.

The n-amyl acetate and tetrachloroethane added to eliminate blushing of coatings may be replaced by other materials such as cyclohexane or dimethyl Cellosolve.

EXAMPLES 7-24 Aluminum foil was dip coated in the manner described in Example 6 to provide coatings having about 1 part of indicator to 5 parts of polymer at from 3 to 5 microns dry film thickness. The more sensitive films had a lower value of marking energy expressed as microampere-sec./ mm. The polymer was vinyl chloride/ vinyl acetate (87/ 13 mol ratio) except where indicated.

Marking energy (microamp- Ex. Coating sec/mm!) 7 Methylene Blue in polyvinyl alcohol 3 8 Methylene Blue 100 9. Methyl Violet 100 10. Fluorsecein chloride IL... Thymol Blue 60 12 Neutral Red 13. Thymolphthalein. 14 Bromocresol Purple 20 15. Bromophenol Blue 20 16 Bromocresol Purple made alkaline by NH 10 exposure. 17. Bromophenol Blue made alkaline by NH 50 exposure.

18..-. Congo Red 10 19 Brilliant Paper Yellow 20 20 Neutral Red (azine).. 80 21- Indophenol Blue 30 22 Salochrome Yellow 30 (hydroxyazo). 50 23- Induline Base B 20 24..-. Nile Blue BXA 10 In general, those acid sensitive indicators with a marking energy of below 10 microampereseconds/mm. preferably below 0.3 microampere-seconds/mmP, are superior for the recording sheets of this invention.

EXAMPLE 25 1.0 by weight Victoria Green WB Base and 10% of a copolymer of vinyl acetate/vinyl chloride and 89 weight percent of acetone was coated onto 1 mil aluminum foil .at a wet coating thickness of 1 mil. The coating was dried to a slightly yellow, clear film (leuco reaction). A 20 frame exposure to an electron beam (19 kv. and -125 'microamperes, 0.3 micron vacuum) produced a green positive image on a clear background.

EXAMPLE 26 1.0% Victoria Green WB Base and 10% of a copolymer of vinylidene chloride and acrylonitrile and 89 percent by weight of acetone was coated onto 1 mil aluminum foil at a wet coating thickness of 1 mil. The coating dissolved in methylene chloride (for the more soluble illwas dried to a dark green color. A frame exposure to dicators) or dispersed by ball milling for four hours or an electron beam (19 kv. and 115-125 microamperes, 0.3 less. Coatings were prepared by knife coating onto a micron vacuum) produced a reddish purple positive metal substrate to provide a dry film thickness of 0.1 image one green background. Rhodamine B Base can mil. Table I sets forth these results. The electron beam be used instead of Victoria Green WB Base. exposure is expressed as fields (6O fields per second).

TABLE I pH for color Electron Beam Exposure at Image Image Before Color After Ex. Indicator change TV rate of frames/sec. Quality Exposure Exposure 27 Crystal Violet 1.8 30 fields (30 kv., 100 amp) Fair Yellow. 28"... Methyl Violet 1. 6 30 fields (30 kv., 160 ramp) .do Do. 29 Malachite Green. Do. 30 Homorosaniline Faint yellow. 31 Red. 32 Blue. 33 Do. 34 Red. 35 Phenolsulione phthalein Yellow 36 Crystal Violet (leuco base)- Blue. 37 Pal-arosaniline (carbinol bas ca. 6 1 field (20 kv., 35 ramp) do Red. 38 Diphenylamine 1 1. 0 30 fields (30 kv., 160 amp).. Fain". Yellow. 39 Methylene Blue 1. 0 .do (i Colorless. 40 Sevron Yellow R fields (30 kV., amp). Green. 41 do Red. 42 30 field Yellow. 43 ..do Do. 44 p-Diphenylamine sulfonic acid, barium salt. Colorless 45 Bindschedlers Green do do Light green Dark green.

1 With oxidation.

The following examples illustrate further indicators and their use in the electron beam recording media of this In the examples of Table II the preferred indicators, invention. The highly halogenated polymeric binder was i.e. amino azo compounds are employed. The highly a copolymer of 87 mol percent vinyl chloride and 13 mol halogenated binder was a copolymer of 88 mol percent percent vinyl acetate, the weight ratio of indicator to 30 vinyl chloride and 12 mol percent vinyl acetate, the binder being 1 to 10. Indicator and binder were either weight ratio of indicator to binder being about 1 to 5.

TABLE IL-AMINO AZO DYES IN ELECTRON SENSITIVE MEDIA Energy to Mark (microamperes-sec. Ex. Indicator at 2,000 volts) Color Response 0H3 A N=N N\ 0. 2 Red on yellow.

C O OH (Methyl Red) N'Hz N Hz D. N: 10 Blue on red, poor solubility.

THF

SOsH SOlH NHz N E.. 2 Blue on red, excellent stability.

CH3 CH3 G..." N=C NHC .2 Red on yellow.

CHr-N-CHa om-N-olar,

H 1 Blue on red.

TABLE II-Continued Energy to Mark (microamperes-sec. Ex. Indicator at 2,000 volts) Color Response I NaS0 N=N-N(OH 5 Redto yellow.

(CHa)zN-N=N-N=NQN(0H3)2 2 Orange on yellow. K NHN=N N=NNHC 3 Orange on yellow.

QN= -C -NHCO 0H; 6 Tan on yellow.

OH;-N=N-N(CHQ2 .2 Red on yellow. N.... (Ilia-- =N--NH2 .03 Purple on yellow.

o-. HO 0 CN=NN(GH3); 1 Red to yellow. P Gino-O- -NH1 .03 Purple on orange.

Q CH3O NH2 .2 Blue on purple.

-sOaN8 R.. H2N-N=N-Q-N=NO-NH, .03 Blue on red.

OH cmo-N=N-Nm .03 Brown on ten. T..... CHaO-C -N=N-8N(C H 40 Orange on yellow.

U Q-N: N\ .02 Purple on yellow.

(mo-ON: G-NH-O-NO; .33 Purple on yellow.

7 W CzHsO-C N=N NH; .2 Brown on tan.

0 111, X. CHaO 3N=N-N H1 2 Pale yellow on ten.

| O CH;

Y CEO-Q NH; .2 Purple on yellow, excellent stability.

Z----- CEO-- OH .2 Red on tan.

I HO N=NOOCH1 zz (GzfinhNO- OCH .2 Yellow on tan.

The following is claimed:

1. A recording process which comprises" (a) exposing to a modulated electron beam a recording media comprising an electrically conductive substrate, and superimposed thereon, a film which is essentially insensitive to visible light and which comprises, as the sole constituent capable of liberating hydrogen halide under exposure to an electron beam, a normally solid, highly halogenated polymer having a molecular weight of at least about 1000 and having at least about 25 weight percent of labile halogen selected from the group consisting of chlorine and bromine, thereby liberating said hydrogen halide, and subsequently b) contacting said film with an acid sensitive indicator capable of changing color at a pH below about 7, thereby causing a visible color change in those areas containing liberated hydrogen halide.

2. A recording process which comprises (a) exposing to a modulated electron beam a recording media superimposed on an electrically conductive backing member, said media comprising a film which is essentially insensitive to visible light and which comprises, as the sole constituent capable of liberating hydrogen halide under exposure to an electron bean, a normally solid, highly halogenated polymer having a molecular weight of at least about 1000 and having at least about 25 weight percent of labile halogen selected from the group consisting of chlorine and bromine, thereby liberating said hydrogen halide, and subsequently (b) contacting said film with an acid sensitive indicator capable of changing color at a pH below about 7, thereby causing a visible color change in those areas containing liberated hydrogen halide.

3. A recording process which comprises exposing to a modulated electron beam a recording media comprising an electrically conductive substrate and, superimposed thereon, a film which is essentially insensitive to visible light and which comprises an acid sensitive indicator capable of changing color at a pH below 7 and, as the sole constituent capable of liberating hydrogen halide under exposure to an electron beam, a normally solid, highly halogenated polymer having a molecular weight of at least about 1000 and having at least about 25 weight percent of labile halogen selected fromthe group consisting of chlorine and bromine, liberating said hydrogen halide to react with said acid sensitive indicator, thereby forming a visible color contrast between the areas corresponding to the exposed and nonexposed portion of said film.'

4. A recording process which comprises exposing to a modulated electron beam :1 recording media superimposed'on an electrically conductive backing member, said recording media comprising a film which is essentially insensitive to visible light and which comprises an acid sensitive indicator capable of changng color at a pH below 7 and, as the sole constituent capable of libcrating hydrogen halide under exposure to an electron beam, a normally solid, highly halogenated polymer having a molecular weight of at least about 1000 and having at least about 25 weight percent of labile halogen selected from the group consisting of chlorine and bromine, liberating said hydrogen halide to react with said acid sensitive indicator, thereby forming a visible color contrast between the areas corresponding to the exposed and nonexposed portion of said film.

References Cited UNITED STATES PATENTS 3,195,110 7/1965 Nail 34()173 3,239,841 3/1966 Henkes 34674 2,630,484 3/1953 Groak 346 X 1 3,245,785 4/1966 Miller et al. 346-74 X 3,370,981 2/1968 Ney et a1. 1l7230 JOSEPH W. HARTARY, Primary Examiner U.S. Cl. X.R. 1l7-1.7 

